In a wireless train communication system, such as the Positive Train Control (PTC) system, a fast-travelling locomotive communicates with base stations and wayside devices (also called track-side) through radio links. A spectrum at 220 MHz has been allocated for the wireless PTC application, where time division multiple access (TDMA) is selected as the underlying media access (MAC) technology to provide a pair of reliable point-to-point TX/RX communication links for a selected locomotive to communicate with an intended base stations. In the PTC system, multiple channels are used by different base stations to segregate the traffic from different locomotives into different channels. This technique can improve the overall network capacity and throughput significantly. In a hypothetic ideal communication system, a radio receiver could use a band-pass filter to only allow in-band signal to pass and to fully reject the signal in adjacent channels. Nevertheless, it is not practical to design such boxcar-type filters. Therefore, while the band-pass filter used in a receiver can substantially suppress out of band signal, there is always a small amount signal leakage from adjacent channels. If the adjacent channel signal is very strong, the leak-through signal from adjacent channels may become substantial and may affect the performance of the intended signal. On the other hand, a transmitter always has out of band leakage where a small amount of signal is radiated outside the allowed or intended spectrum, which may be caused by system phase noise and/or other system impairments. Accordingly, the issue of adjacent channel interference always exits and should be taken into consideration for reliable system design.
In the PTC system, the base stations are stationary and the channels may be pre-assigned to minimize potential interference among base stations. However, the locomotives are mobile. When a locomotive, transmitting at an adjacent channel associated with a base station, passes by the base station, the transmitted signal from the locomotive may be perceived as a strong interference signal at an adjacent channel. Therefore, the intended signal to be received by the base station may be temporarily affected by the transmitted from the locomotive. Similarly, the signal from the base station may also affect the intended signal to be received by the locomotive when the locomotive is close to the base station if the base station is transmitting at an adjacent channel. In the PTC system, the interference is intensified because train control network usually comprises base stations with highly overlapped area for redundancy purposes. Therefore, a good system design should dynamically assess potential interferences from adjacent channels and allow a device to transmit only if good signal quality is available.